Calculation of Noise Reduction Rating

The NRR is a single-number rating (a simplified number from a broad audible frequency) which is required by law to be shown on the label of each hearing protector sold in the United States. The NRR is specified by 40 CFR (Code of Federal Regulations) Part 211, Product Noise Labeling, Subpart B – Hearing Protective Devices. It is independent of the noise spectrum in which it is applied.

Calculating the NRR:8000f = 1 2 5where L at frequency overall level of 107.9 dBC, and APV mean attenuation value minus 2 standard deviations at frequency deviations accounts for 98% of the variance in a normal distribution).The Hearing Conservation Amendment to the Occupational Noise Standard (OSHA, 1983) describes six methods for using the NRR to determine a worker’s protected A-weighted noise exposure. These methods vary according to the instrumentation and parameters used to determine the unprotected noise levels. However, they can be summarized into two basic formulas, depending on whether unprotected exposure levels were measured on a C-weighted or an A-weighted scale.

Using the NRR:

For C-weighted measurements:

NRR = 107 .9 dBC – 10 log

S 10 0.1(LA

f-APVf98) - 3 dB.

The equation can be broken down into the steps shown in Table A.1.1 on the following page. The NRR assumes a pink noise with octave-band levels of 100 dB (line 1). The corrections for the C-weighting scale (line 2) are then subtracted to compute unprotected Cweighted octave-band levels at the ear (line 3).

These octave-band levels are logarithmically summed to obtain the overall sound level in dBC at the unprotected ear; this value is the first term of the equation (i.e., 107.9 dBC).

The corrections for the A-weighting scale (line 4) are then subtracted from the pink-noise octave-band levels (line 1) to compute the Aweighted octave-band levels at the ear (line 5).

The average attenuations (line 6) minus twice the standard deviations (line 7) are subtracted from the A-weighted octave-band levels to compute the protected A-weighted octaveband level at the ear. Note that the attenuation data for 3000 and 4000 Hz and for 6000 and 8000 Hz are averaged to derive the attenuations at 4000 Hz and 8000 Hz. Note also that the standard deviations for 3000 and 4000 Hz and 6000 and 8000 Hz are summed (in lieu of multiplying by 2) to calculate the total standard deviation for 4000 Hz and 8000 Hz. The protected A-weighted octave-band levels at the ear are then logarithmically summed to calculate the overall protected A level. This is the second term of the equation.

The NRR is computed by subtracting 3 dB from the difference between the unprotected C-weighted and the protected A-weighted levels at the ear.

The values of sound attenuation used for calculation of the NRR are determined in accordance with ANSI S3.19-1974, “American National Standard for the Measurement of Real-Ear Hearing Protector Attenuation and Physical Attenuation of Earmuffs.” The experimenter-fit method must be used; that is, the experimenter (not the test subject) must fit the hearing protector onto the head or into the ear of each test subject for each occluded test. Mean attenuations and standard deviations are calculated in accordance with the standard. The NRR is then computed from the mean attenuations and standard deviations according to the following equation

protected dBA = unprotected dBC – NRR

where the protected dBA and the unprotected dBC are 8-hour time-weighted averages determined according to the Occupational Noise Standard. This method is how the NRR was designed to be used. For example, if a protector has an NRR of 17 dB and it is used in an environmental noise level of 95 dBC, the noise level entering the ear could be expected to be 78 dBA [95 - 17 = 78] “or lower in 98% of the cases if the protector is worn according to manufacturers specification.

For A-weighted measurements:

protected dBA = unprotected dBA -[ NRR - 7 ]

where, again, the protected and unprotected dBA are 8-hour time-weighted averages calculated according to the Occupational Noise Standard. This is an adaptation for those whose device does not have C-weighting capabilities. The 7 dB correction factor is used to account for the de-emphasis of low-frequency energy inherent to the Aweighting scale. So, for instance, if a protector has an NRR of 17 dB and it is used in an environmental noise level of 95 dBA, the noise level entering the ear could as high as 85 dBA [95 - (17 - 7) = 85] or less in 98% of the cases.

NIOSH recommends derating the NRR by a multiplicative factor of 75%, 50% and 30% for earmuffs, formable earplugs and all other earplugs, respectively. This variable derating scheme, as opposed to OSHA’s straight derating scheme, considers the real-world performance of different types of hearing protectors (NIOSH, 1998). Also, the NIOSH derating scheme does not affect the 7 decibel dBC to dBA correction which OSHA derates by 50%. This compendium uses the NIOSH derating of the NRR when searching for protectors based on user input of noise exposure levels in dBA, dBC, or octave band levels.